Anti-Infective Agents
Abelson-family tyrosine kinases (ATKs) are host targets that can be inhibited by small molecules (ATKis) to create anti-cancer and anti-infective therapeutics. When applied to treat infectious diseases in vitro and in vivo, more than 20 human bacterial viral, fungal and parasitic pathogens have been shown to be susceptible to ATK inhibition. Host ATKs are co-opted by a pathogen to enter, reproduce or exit host cells, accounting for the antimicrobial effects of ATKis. Because the targets of these therapeutics to treat infectious disease overlap with the targets for certain types of cancer, the same medications for infectious disease can be applied to treat cancer at the same dose.
For infectious disease, this strategy has been applied to treat the cause of Progressive Multifocal Leukoencephalopathy (PML) a fatal brain infection that arises in chronically immunosuppressed populations including people with HIV1 infection, patients on chronic immunosuppressive therapy such as corticosteriods for organ transplant, patients with cancer and/or autoimmune diseases (such as rheumatoid arthritis, psoriasis, and lupus erythematosis), and patients on therapies that depress the immune response (e.g., efalizumab, belatacept, rituximab, natalizumab, infliximab, among others). PML is a fatal condition caused by the lytic infection of JC polyomavirus (JCV) in the brain. However, the brain-infective form of the virus is not acquired by transmission from an infected patient. Instead, brain-infective JCV is formed by genomic rearrangement of the non-pathogenic form of the virus that resides outside of the brain in a persistently infective state within a patient. This non-pathogenic, or archetype JCV, which is detected in the kidney, genitourinary tract and bone marrow, is kept in check by the host cellular immune response. The transformation to the brain-infective form occurs when a patient undergoes a sustained disruption of cellular immunity, which could be caused by either immune-suppressing drugs to treat autoimmune disease (e.g. MS) or by development of clinical AIDS following HIV infection. In the context of immunosuppression, JCV rearranges its non-coding control region (NCCR), followed by mutations in the viral capsid protein VP1. Rearranged NCCR is thought to enable the virus to replicate in a wider range of cells, while the subsequent mutations in VP1 enable viral entry through a broader range of receptors found on the surface of cells outside the genitourinary tract. Neither the pathway to the brain nor the carrier enabling JCV to enter the brain is definitively known, but it is a slow process that takes years to complete. A characteristic marker of PML is the appearance of viral DNA in the cerebrospinal fluid (CSF). In a typical clinical case, a patient is negative for JCV DNA within 2 months of a diagnosis, after which JCV DNA is readily found in the CSF. Thus, the process of central nervous system (CNS) entry is very slow, but once virus enters the CNS, progression to disease is relatively rapid.
Lytic infection of JCV in the brain causes irreversible damage to neural tissue. Thus, it would be desirable to clear JCV from a patient early in the course of treatment. An ATKi could be used alongside therapies like Tysabri for MS, to clear JCV at the initiation of an immune-suppressing treatment.
Proof-of-concept trials related to a PML antiviral program to treat JCV infection with the marketed drug Gleevec, a well-known anticancer Abl-kinase inhibitor, were conducted. Gleevec was very useful for defining the mechanism of action, but it rapidly became clear that the human steady-state (SS) concentration of Gleevec cannot sustain an efficacious dose for an antiviral purpose (Table 1). The steady-state trough concentration, CminSS, is just 2.3-fold higher than the EC50 of Gleevec against JCV in cell culture (Table 1). Typically, this ratio should be 4- to 9-fold above the EC50 value for a safe and effective antiviral agent.
Improved agents for treating JCV and other infectious agents are needed.
Anti-Cancer Agents
Chronic Myeloid Leukemia (CML) is a myeloproliferative neoplasm with an incidence of 1-2 cases per 100,000 persons, and accounts for ≈15% of newly diagnosed cases of leukemia in adults. Pathogenesis of CML is linked to the fusion of the Abelson murine leukemia (ABL) gene on chromosome 9 with the breakpoint cluster region (BCR) gene on chromosome 22, resulting in expression of fusion protein, termed BCR-ABL. BCR-ABL is a constitutively active tyrosine kinase that promotes growth and replication through downstream pathways such as RAS, RAF, JUN kinase, MYC, and STAT. The consequences of BCR-ABL expression create a cytokine-independent cell cycle with aberrant apoptotic signals in response to cytokine withdrawal. The development of small molecule ATKis that potently interfere with the interaction between BCR-ABL and ATP were shown to block cellular proliferation of the malignant clone. This “targeted” approach was found to dramatically alter the natural history of the disease, improving 10-year overall survival (OS) from ≈20% to >80%.
Three such targeted therapies have been approved for first line treatment of CML: imatinib (Gleevec®), nilotinib (Tasigna®) and dasatinib (Sprycel®). Gleevec, the first of these agents to reach market, displayed a remarkable 81% event-free survival rate and a 93% overall survival rate when CML-only related deaths were considered. But, 8-year follow-up studies revealed that only 55% of patients remained on therapy, indicating that additional options were needed to handle treatment failure and improve tolerability of Gleevec as a chronic medication. Treatment failures were often linked to the development of Gleevec resistance arising from secondary mutations in the Abl-kinase domain of the fusion protein, resulting in a loss of Gleevec potency and relapse of disease. Sprycel, a dual Abl- and SRC-kinase inhibitor, was the second approved agent and a much more potent inhibitor of BCR-ABL. Sprycel induces more rapid responses and suppression of fusion protein expression at much earlier timepoints than Gleevec. But, as a SRC inhibitor, significant side effects counterbalanced the value of the enhanced response profile that Sprycel displayed. Tasigna was developed to directly address Gleevec resistance, and, early in its development, demonstrated potent inhibitory activity against many clinically relevant BCR-ABL mutants that no longer responded to Gleevec therapy (Table 3). Like Sprycel, Tasigna was as much as 20× more potent as an inhibitor of BCR-ABL relative to Gleevec (Table 3). However, Tasigna's potency was accompanied by a side effect profile with severe adverse event frequency similar to Sprycel, limiting its utility as a chronic medication.
Thus, despite the success of targeted therapies against BCR-ABL, comorbidities and toxicities are a dominant issue in the use of these frontline ATKis for some patients. Patients at risk of developing pleural effusions, for example, such as patients with a history of lung disease (e.g., COPD), cardiac disease (e.g., congestive heart failure or pulmonary arterial hypertension) or patients with uncontrolled hypertension, make Sprycel a poor choice. Sprycel also inhibits platelet function, so patients taking anticoagulants could be at risk for bleeding complications while on Sprycel. Tasigna is associated with hyperglycemia and therefore could be detrimental if used in patients with uncontrolled diabetes. Tasigna may also prolong the QT interval and therefore may be contraindicated in patients with cardiac complications, although longer-term follow-up studies appear to be needed to confirm this observation. The non-linear accumulation of Tasigna that occurs when taken in the context of a fatty diet also places patients at risk for reaching severe dose-limiting toxicities, requiring fasting before and after dosing with Tasigna for 2 hours; as Tasigna is given 2×/day, this means patients on Tasigna are fasting as much as 8 hours out of every 24. By contrast, while Gleevec is associated with some severe AEs (e.g., leukopenia and cytopenia), its most prominent side effect is peripheral edema, which can be medically managed. Ponatinib, the most recent ATKi to reach market and which addresses nearly all Gleevec resistance, including the T315I ‘gatekeeper’ mutation, was subsequently found to cause a severe blood clotting syndrome and a narrowing of blood vessels after 24 months of therapy. As a result, ponatinib's utility has been severely restricted as a third-line treatment when no other options exist. Bosutinib, another recently approved Abl-SRC dual inhibitor, is considered second-line in the context of imatinib-resistance with potency that is similar to Sprycel.
Similarly, GastroIntestinal Stromal Tumors (GIST), the most common mesenchymal tumors originating in the digestive tract, have a characteristic morphology and are generally positive for CD117 (c-kit) and are primarily caused by activating mutations in the KIT or PDGFRa, both protein kinases in the Abelson-kinase family which are susceptible to treatment with ATKis. Just as found in the case of BCR-Abl associated cancers, KIT and/or PDGFRa associated GIST treated with frontline TKIs like imatinib can eventually develop resistance to therapy through the formation of secondary mutations in either KIT or PDGFRa; imatinib also displays poor response rates in KIT exon 9 and wildtype associated tumors, both of which are more responsive to higher affinity ATKis like sunitinib and regorafenib. Given the lower overall response rates of sunitinib and regorafenib relative to imatinib in front line therapy, the development of new agents with the broadest application in the manner of imatinib offers a likelihood of treatment success that will exceed the higher affinity agents like sunitinib and regorafenib.